1. Field of the Invention
This invention relates to control circuitry for communication devices and specifically to control circuitry for performing signal protocol exchanges to establish a communications link between two radio stations.
2. Description of the Prior Art
Mobile telephone equipment has existed for some time and enables a mobile radio station including a transmitter and receiver to communicate with a base station which also includes a transmitter and receiver. Mobile telephones more specifically allow the interaction of a number of separate mobile radio stations with a single base station over several different frequency channels. The mobile stations establish a communications link with the base station by first determining when the base station is available and second by performing a handshake or signal protocol exchange with the base station to establish the identification of the mobile unit calling and to establish the number to be called. Several types of signal protocols exist for mobile telephone equipment. One such system is the Improved Mobile Telephone Services (IMTS). The interaction of the IMTS mobile units with base stations is outlined in the Bell System Technical Reference, PUB 43301 entitled "Bell System Domestic Public Land Mobile Radio Services-Interface Specifications For Customer-Provided Mobile Terminals".
Usually, to interface with a base station, the mobile station must have a full duplex capability, i.e. be able to transmit and receive at the same time. This capability enables the mobile station to receive protocol signals from the base station and transmit response signals in accordance with the appropriate protocol. However, half duplex equipment, i.e. equipment that can only transmit or receive at any instant, is much cheaper to manufacture. Half duplex equipment has been used in place of full duplex equipment with base stations that require signal hand shaking or protocol exchanges by sending the required response signals at estimated time intervals to simulate reception and response to protocol signals from the base radio station. If the time intervals are proper, the mobile station will look to the base station as if it is responding properly to the signal protocol. In fact, the signals that are being transmitted by the mobile stations are in accordance with time estimates designated by the circuit designer for simulating the signal protocol exchange required by the base station. Unfortunately, not all base station signal protocols are uniform. These base station protocols vary from base station location to base station location. If the time interval estimates do not fall within the base stations signal protocol limits, the communications link between the base station and the mobile station will not be established. Therefore, the time intervals for the mobile units may have to be adjusted each time the mobile unit interfaces with a different base station. However, one advantage of mobile communication, i.e. the ability to travel and communicate with different base stations is lost.
It is the object of the present invention to provide a control circuit for use in mobile radio units that provides time intervals for half duplex systems that adapt to the time interval protocol requirements of individual base stations.